1. Field
This disclosure relates generally to pulsed lasers and machining materials using high repetition rate pulsed lasers.
2. Description of the Related Art
Composite materials are engineered materials having, for example, at least two different materials with significantly different physical and/or chemical properties. The physical and/or chemical properties of the different materials may remain substantially separate, distinct, or distinguishable at a macroscopic or microscopic scale within the composite material. By way of example, the material may be composed of reinforcement (e.g., fibers, particles, flakes, and/or fillers) embedded in a matrix (e.g., polymers, metals, and/or ceramics). The matrix holds the reinforcement to form the desired shape while the reinforcement improves the overall mechanical properties of the matrix. Composite materials are used in variety applications, due to their combination of different material properties, such as strength and light weight, flexibility and rigidity, chemical and fire resistance among others.
Portions of a printed circuit board (PCB) may be fabricated with a composite material so as to provide mechanical strength, satisfy weight limitations, flexibility, and/or rigidness as well as thermal and chemical resistance. PCBs used in high speed electronics may include low-k dielectrics, expanding the functionality of the composite material. Low-k material can include material that has a dielectric constant that is less than the dielectric constant of silicon dioxide. For example, low-k material can include dielectric materials such as doped silicon dioxide, polymeric dielectrics, etc. The desirability of “green technology,” and the associated use of lead-free and hazard-free material, is also a consideration. Therefore, challenges are apparent in fabrication of various PCB designs.
A glass fiber matrix, a carbon fiber matrix, or metallic structure may be used for reinforcing the mechanical structure of a composite material, where the structure is hosted in polymers or organic resin to mechanically stabilize the form. For instance, Garolite, which combines glass fiber mesh filled with epoxy resin, is a commonly used substrate for electronics PCB.
Composite materials (e.g., that can be used in PCB substrates) can include, for example, National Electrical Manufacturers Association (NEMA) grades: FR-1, FR-2, FR-3, FR-4, FR-5, FR-6, G-10, and composite epoxy materials including CEM-1, CEM-2, CEM-3, CEM-4, CEM-5, etc. Composite materials can include woven fiberglass cloth with an epoxy resin binder, resin bonded paper, cellulose paper having a woven glass fabric surface, woven glass and epoxy, cotton paper and epoxy, matte glass and polyester, woven glass and polyester, and glass reinforced epoxy laminates. In some types of PCBs, conducting layers of the PCB are made of a thin, electrically conductive foil or layer (e.g., copper) and insulating layers of dielectric materials are laminated with epoxy resin pre-impregnated (prepreg) composite fibers. In some cases, dielectric materials can include polytetrafluoroethylene (PTFE or Teflon®), polymide, FR-1, FR-4, CEM-1, and/or CEM-3. In some cases, prepreg materials can include FR-2, FR-3, FR-4, FR-5, FR-6, G-10, CEM-1, CEM-2, CEM-3, CEM-4, and/or CEM-5. For example, certain PCBs comprise an insulating FR-4 layer on which a thin layer of copper foil is laminated (on one or both sides of the FR-4 layer). Thickness of the electrically conductive layer (e.g., copper foil) can be in a range from about 10 μm to about 100 μm (for example, about 18 μm or about 35 μm). Other conductive layer thicknesses can be used. Thickness of the PCB substrate can be in range from about 500 μm to about 1500 μm (e.g., about 800 μm or about 1000 μm). Other substrate thicknesses can be used (e.g., about 30 μm to about 250 μm).
In some cases, a PCB can comprise between about one and twenty conductive layers (e.g., copper) laminated with insulating (e.g., dielectric) layers. In some cases, the PCB can comprise a multi-layer PCB in which a plurality of PCB layers are bonded together. In some cases, PCBs can be coated with one or more substances (e.g., wax, silicone rubber, polyurethane, acrylic, or epoxy) to inhibit corrosion and electrical shorting.
Physically separating, for example dicing, of a PCB comprising such composite material is not without challenges. Conventional mechanical dicing has been the most common method, but with significant drawbacks. When the material thickness becomes less than approximately 1000 μm, cutting with a mechanical saw may cause various problems such as chipping, fraying, tearing, and delamination. A high speed PCB may have a substrate with a thickness of about 125 μm, and a substrate thickness may be in the range from about 30 μm to about 250 μm. Moreover, special tooling may be required for mechanical dicing of thin PCBs, especially if an irregular shape is needed. Chemical processing bears high environmental cost and is considered, at the same time, non-green in many industries.